Screen printing has been used in an electronic component mounting step as a method for printing a substrate with paste, like cream solder and a conductive paste. The method includes setting a substrate on a screen mask with pattern holes opened and aligned with locations of print targets and slidably moving a squeegee over the screen mask supplied with paste, thereby printing the substrate with the paste through the pattern holes. During screen printing, the paste fed over the screen mask is consumed as a result of repeated performance of squeegeeing action for slidably moving the squeegee, which decreases an amount of paste remaining on the screen mask. The paste is replenished at timing when the amount of residual paste falls short of a specified amount of paste.
In order to adequately set the paste replenishment timing, a hitherto-known screen printer has a paste detection function for detecting whether or not a specified amount of paste still exists on a screen mask (see; for instance, Patent Document 1). According to the related art described in connection with the patent document, an amount of paste remaining on the screen mask is detected on the basis of whether or not a sensor placed at a fixed position previously set as a point of detection detects paste.